In recent years, multilayer optical films have been developed which have a wide variety of interesting properties. Such films are described, for example, in U.S. Ser. No. 08/359,436, now abandoned. In the continuing development of these films, particular attention has been paid to maintaining the integrity of the multilayer structure in these films during manufacture, and to promoting interlayer adhesion between the individual layers. Good inter-layer adhesion between co-extruded layers in multilayer optical films is desirable to reduce the possibility of delamination during post-processing and end use. These aspects are crucial to the optical and mechanical properties of the film.
Material or layer characteristics that effect inter-layer adhesion and layer integrity include, but are not limited to, the relative affinity of the materials for each other (characterized by quantities such as solubility parameters, interaction parameters or interfacial tension), the ability of these materials to interact by chemical reaction, the roughness of the inter-layer interface (e.g., both local concentration fluctuations in the interfacial zone and the planarity of this zone), the broadness of the average concentration profile of the materials across the interfacial zone, the molecular weight distribution and average (or intrinsic) viscosity of the materials, the melt viscosities, normal stresses, the so-called "entanglement" weights of the materials, and the mobilities or diffusion coefficients of the materials. If the materials chemically react across the interface, inter-layer adhesion may be promoted by the creation of crosslinks or other forms of covalent bonding, including the formation of copolymers.
Inter-layer adhesion can also develop, and layer integrity can diminish, without chemical reaction. The higher the relative affinity between the materials of adjacent layers, the higher the inter-layer adhesion. If the affinity is sufficiently high, the materials become miscible, and the rate of interdiffusion then determines the final structure. When a multilayered structure is desired, excessive interdiffusion between miscible materials in adjacent layers can destroy the layered structure, and thus needs to be limited. Higher molecular weight can reduce interdiffusion, and can also limit ultimate miscibility.
If the affinity between the materials of adjacent layers is insufficient to cause miscibility, then an interfacial zone will develop over which the concentration of one material varies from nearly its pure value to nearly zero. Since materials are rarely perfectly immiscible, the concentration of the material in its original layer is likely to be somewhat less than its original value, and the concentration of the material in layers originally of other materials is likely to be somewhat greater than zero. In cases of partial miscibility, the initially pure material layers may tend to evolve toward thermodynamic phases that maintain a majority of the original material but also contain a substantial portion of material from neighboring layers. Partial miscibility may be the result of the lower molecular weight fractions inherent in, or added to, the polymeric material. As the affinity increases, the effective width of the interfacial zone increases and the ultimate purity of the layers decreases. Changing the purity of the layers can alter their behavior under subsequent processing (e.g., orientation and crystallization under draw) thus changing the optical and mechanical properties of the final film.
The width of the interfacial profile can also increase inter-layer adhesion. For example, a broader interface may be able to dissipate fracture energy more effectively, giving rise to increased strength. Moreover, weld strength correlates positively with the degree of inter-entanglement between layers. As the interfacial breadth decreases relative to the radius of gyration of a polymeric coil of the entanglement molecular weight, the weld strength is expected to decrease. Ultimate inter-entanglement, and thus inter-layer adhesion, between layers can also be increased by increasing the average molecular weight of the materials if the interfacial zone is broad enough relative to the size of the molecular coils. However, increasing the molecular weight can also slow interdiffusion and prevent the establishment of equilibrium. Finally, the interfacial profile necessarily establishes a gradient in the optical and mechanical properties throughout this interlayer region, thereby altering the properties of the construction in toto. As the interfacial zone width approaches the layer thickness, the layer integrity deteriorates with severe effects on the film properties. Thus, processing conditions and design considerations that effect these inter-layer characteristics are also clearly relevant.
Temperature and residence time can greatly affect the broadness of the average concentration profile of materials across the interfacial zone by affecting both interdiffusion and chemical reaction (where possible) between layers. Initially, the individual layers make contact in the feedblock and eventually weld in, or downstream of, this feedblock. Higher temperatures can increase the mutual welding and diffusion process which establishes the interfacial zone in situ. If the materials do not react and are immiscible, then there exists an equilibrium average concentration profile with some average interfacial zone width.
For given materials of given mobilities, processing temperatures and residence times determine how closely to equilibrium the interfacial zone can reach before web quenching at the casting wheel. If the materials can react, there exists a quasi-equilibrium at a given level of chemical reaction. As the reaction proceeds, this equilibrium can shift to a broader equilibrium profile. This latter case may include, but is not limited to, PEN:coPEN systems and other polyesters in which transesterification reactions can create co-polymers of the two initial materials in situ within the interfacial zone. The actual final interfacial profile between the layer compositions and the layer integrity are then the result of coupled diffusion and chemical reaction. Henceforth the term "interdiffusion" will imply both coupled processes. The interfacial profile resulting from interdiffusion can vary from a sharp boundary between distinct material layers through any intermediate stage up to and including profiles between layers at phase equilibrium. Because interlayer adhesion and layer integrity required for optical performance often behave oppositely in quality with degree of interdiffusion, processes may sometimes optimize at some intermediate interfacial profile.
In another example of process effects, melt train temperatures and drying conditions can affect the molecular weight distribution of the materials. Moreover, temperature and shear rate can effect the component layer viscosities which can, in turn, effect flow stability and inter-layer surface roughness. In some cases, relatively low levels of deliberate flow instability could conceivably enhance inter-layer adhesion without destroying the multilayer stack construction. Finally, process design considerations can also be important. For example, layers are compressed within the feedblock and then again within the die. An inter-layer profile established in the feedblock could be compressed in the die, requiring further interdiffusion to re-achieve the equilibrium interfacial width. Controlling residence times through the various portions of the melt train can control the degree of interdiffusion.
The effective index differentials of multilayered optical films are often observed to vary somewhat from the values predicted from the corresponding monolithic films. This variance is most pronounced in the thin optical layers (that is, those layers which are tuned to the blue region of the spectrum, or layers that are intentionally made less than 1/4 wave thick for other regions of the spectrum). This phenomenon is sometimes attributed, at least in part, to interlayer diffusion. By way of illustration, as shown in FIG. 1, the index differential at 550 nm for monolithic films of PEN and coPEN stretched to a 6:1 ratio is about 0.25. However, from the measured reflectance of several 1/4 wave optical stack samples of PEN/coPEN multilayer film, the effective index differential of the actual thin optical layers is somewhat smaller, typically about 0.20, and has been observed to drop to as low as 0.05. These variations in effective index differential adversely affect the optical properties of the film, with the result that reflective polarizers and other optical films made with these materials often attain only a fraction of their theoretical performance.
Various references describe films having thin optical layers or methods for making such films. Representative examples include U.S. Pat. Nos. 3,711,176 (Alfrey, Jr. et al.), 3,773,882 (Schrenk et al.), 3,884,606 (Schrenk), 5,126,880 (Wheatley et al.), 5,217,794 (Schrenk), 5,233,465 (Wheatley et al.), 5,269,995 (Ramanathan et al.), 5,316,703 (Schrenk), 5,389,324 (Lewis et al.), 5,448,404 (Schrenk et al.), 5,540,978 (Schrenk), and 5,568,316 (Schrenk et al.). However, to date, the conditions giving rise to variations in effective index differential have been only poorly understood, and no methods have been provided in the art for controlling such variations, particularly in films with thin optical layers. It is thus an object of the present invention to provide such a method.